FIGURE 7-18 How NIRS Works (A) Light injectors (red) and detectors (blue) are distributed in an array across the head. (B) Light injected through the scalp and skull penetrates the brain to a depth of about 2 centimeters. A small fraction of the light is reflected and captured by a detector on the scalp surface. Light is reflected from as deep as 2 centimeters but also from the tissue above it, as illustrated by the banana shape of the curves.

Information from L. Spinney (2005). Optical topography and the color of blood, The Scientist, 19, 25–27.
Hitachi, Ltd. Research & Development Group. Photo by Atsushi Maki